The present invention generally relates to an apparatus and a method for preparing a backside-ground wafer for testing and more particularly, relates to an apparatus and a method for preparing a backside-ground wafer for testing by assembling a calibration wafer to the backside of the backside-ground wafer such that the total thickness is within the test machine tolerance for a wafer thickness.
A backlapping or backside grinding process is frequently used in semiconductor processing for thinning a wafer in order to achieve a smaller thickness, planarity and parallelism of the top and bottom surfaces of a wafer. Since a wafer polishing process can only remove a maximum thickness from a wafer of about 5 xcexcm, a wafer polishing process cannot be used effectively to reduce the thickness, or to correct the curvature of a wafer and to achieve parallelism of the surfaces. The backlapping process, or the backside grinding process is thus used before a wafer polishing process to achieve major thickness reductions. The wafer lapping process is normally carried out in a lapping apparatus which may be used for a single-side lapping or for a double-side lapping.
In a typical backside grinding process, a thickness of the silicon wafer of about 10 mil (or about 250 xcexcm) is removed from the backside of the wafer. For instance, for a six-inch wafer, a reduction of thickness from about 25 mil to about 15 mil, while for an eight-inch wafer, a reduction from about 29 mil to about 19 mil.
While a silicon wafer is normally inspected for defects or for thickness and optical properties of the wafer before the backside grinding process, there are circumstances where after a backside of a wafer is already removed by a grinding process, further tests for either defect or for thickness and optical properties may become necessary. Problems occur when such backside-ground wafers no longer fit in a test machine that was designed for holding a wafer that has full thickness, i.e. a thickness prior to the backside grinding process. For instance, a KLA defect inspection machine would hold a six-inch wafer within a thickness specification of 25 milxc2x11 mil, or hold an eight-inch wafer within a thickness specification of 29 milxc2x11 mil. A backside-ground wafer thus cannot be mounted in such an inspection machine and be tested. This presents a serious problem for a fabrication facility when a wafer, after a backside grinding process, is returned by a customer for further testing of defects.
It is therefore an object of the present invention to provide a method for preparing a backside-ground wafer for testing that does not have the drawbacks or shortcomings of the conventional methods.
It is another object of the present invention to provide a method for preparing a backside-ground wafer for testing by laminating a calibration wafer to the backside-ground wafer for mounting into a test machine.
It is a further object of the present invention to provide a method for preparing a backside-ground wafer for testing by bonding a calibration wafer to the backside of the backside-ground wafer by at least three drops of water applied thereinbetween.
It is another further object of the present invention to provide a method for preparing a backside-ground wafer for testing by mounting a calibration wafer to the backside of the wafer through the capillary reaction of at least three drops of water applied therein-between.
It is still another object of the present invention to provide a method for preparing a backside-ground wafer for testing by mounting a calibration wafer to the backside-ground wafer in a specially designed wafer mounting block that has a slanted surface.
It is yet another object of the present invention to provide an apparatus for mounting a backside-ground wafer to a calibration wafer which consists of a slanted block having a slant angle measured from a horizontal plane between about 10xc2x0 and about 30xc2x0.
It is still another further object of the present invention to provide an apparatus for mounting a backside-ground wafer to a calibration wafer by using a slanted block that is formed of a rigid material such as aluminum or a rigid plastic.
In accordance with the present invention, a method and an apparatus for preparing a backside-ground wafer for testing are provided.
In a preferred embodiment, a method for preparing a backside-ground wafer for testing can be carried out by the steps of first providing a calibration wafer that has a pattern formed of an insulating material on a top side; applying at least three drops of water with each one sufficiently apart from the others on the top side of the calibration wafer; providing a backside-ground wafer that has a ground backside and a front side to be tested; and mating the ground backside of the backside-ground wafer to the top side of the calibration wafer intimately with the at least three drops of water therein-between.
The method for preparing a backside-ground wafer for testing may further include the step of providing a calibration wafer with an oxide pattern formed on a top surface, or the step of providing a calibration wafer with an oxide pattern that has a thickness of at least 2000 xc3x85 formed on a top surface.
The method may further include the step of providing a calibration wafer that has a silicon nitride pattern formed on a top surface, or the step of applying at least five drops of water with each one sufficiently apart from the others on the top side of the calibration wafer. The method may further include the step of forming the pattern of an insulating material on the calibration wafer to a thickness of at least 3000 xc3x85.
The present invention is further directed to an apparatus for mounting a backside-ground wafer to a calibration wafer for testing which includes a slanted block formed of a rigid material that has a top surface with a slant angle measured from a horizontal plane between about 10xc2x0 and about 30xc2x0, the top surface generally has a rectangular shape; a stopper of linear, elongated shape that has a height of at least 5 mm fixedly positioned at and parallel to a bottom edge of the top surface of the slanted block; and two guiding strips of linear, elongated shape which has a height of at least 5 mm fixedly positioned spaced-apart and parallel to two sides of the top surface, the two guiding strips are spaced-apart by a distance that is at least a diameter of the calibration wafer.
In the apparatus for mounting a backside-ground wafer to a calibration wafer, the two guiding strips are spaced-apart by a distance of at least 202 mm, or by a distance of at least 302 mm. The apparatus may further include an annular-shaped recess in the top surface of the slanted block that is at least 5 mm deep with a diameter of at least 202 mm, or an annular-shaped recess in the top surface of the slanted block that is at least 5 mm deep with a diameter of at least 302 mm. The slanted block may be formed of aluminum, or of a rigid plastic.
The present invention is still further directed to a method for preparing a backside-ground wafer for testing by using a wafer mounting block which includes the operating steps of providing a wafer mounting block that includes a slanted block formed of a rigid material that has a top surface with a slant angle from a horizontal plane between about 10xc2x0 and about 30xc2x0, the top surface generally has a rectangular shape; a stopper of linear, elongated shape that has a height of at least 5 mm fixedly positioned at and parallel to a bottom edge of the top surface of the slanted block; two guiding strips of linear, elongated shape that have a height of at least 5 mm fixedly positioned spaced-apart and parallel to two sides of the top surface, the two guiding strips are spaced-apart by a distance that is at least a diameter of the calibration wafer; positioning a calibration wafer that has a pattern formed of an insulating material on a top side of the top surface of the slanted block in-between the stopper and the two guiding strips; applying at least three drops of water with each one sufficiently apart from the others on the top side of the calibration wafer; providing a backside-ground wafer that has a ground backside and a front side to be tested; mating the ground backside of the backside-ground wafer to the top side of the calibration wafer intimately with the at least three drops of water therein-between; and removing a calibration wafer/backside-ground wafer assembly that is bounded together from the wafer mounting block.
The method for preparing a backside-ground wafer for testing by using a wafer mounting block may further include the steps of conducting a test on the front side of the backside-ground wafer; heating the calibration wafer/backside-ground wafer assembly at a temperature of at least 100xc2x0 C. for a time of at least 3 min; and separating the backside-ground wafer from the calibration wafer.
The method for preparing a backside-ground wafer for testing may further include the step of providing a calibration wafer with an oxide pattern formed on a top side, or with an oxide pattern that has a thickness of at least 2000 xc3x85 formed on a top side. The method may further include the step of providing a calibration wafer with a silicon nitride pattern formed on a top side. The method may further include the step of applying at least five drops of water with each one sufficiently spaced-apart from the others on the top side of the calibration wafer. The method may further include the step of forming the pattern of an insulating material on the calibration wafer to a thickness of at least 3000 xc3x85.